Add workflow permissions

[eholk]

Potential fix for https://github.com/microsoft/stackfuture/security/code-scanning/2

To address this issue, we should add the minimal required permissions key to the workflow. Since neither the build nor the miri job requires anything beyond reading repository contents (for checking out code to build/test), we set contents: read as the global permissions. The most efficient and maintainable fix is to add the permissions property at the root of the workflow file (just after the name: and before the jobs section), so it applies to all jobs. No changes to imports or job steps are needed.

Specifically, edit .github/workflows/rust.yml:

• Insert a top-level permissions: block directly after the name: Rust line, prior to the on: block.
• Set contents: read within this block.

No other code, imports, methods, or definitions are required for this correction.

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